Liquid explosive for well fracturing

ABSTRACT

A CAP INSENSITIVE LIQUID EXPLOSIVE IS DESCRIBED WHICH IS PARTICULARLY USEFUL FOR FRACTURING A FORMATION CONTAINING A NETWORK OF NARROW FISSURES ADJACENT A WELL BORE IN ORDER TO BRING IN A WELL OR TO INCREASE ITS PRODUCTIVITY, WHICH INCLUDES A NITROPARAFFIN COMPOUND, PREFERABLY NITROMETHANE, CAPABLE OF DISSOLVING SUBSTANTIAL AMOUNTS OF HIGH EXPLOSIVE COMPOUNDS, AND ONE OR MORE OF CERTAIN HIGH EXPLOSIVE COMPOUNDS DISSOLVED THEREIN. THE HIGH EXPLOSIVE COMPOUNDS ARE OF A KIND AND ARE PRESENT IN AN AMOUNT CAPABLE OF RENDERING THE LIQUID EXPLOSIVE SUFFICIENTLY DIAMETER INSENSITIVE TO PERMIT PROPAGATION OF AN EXPLOSION THROUGHOUT A SUBSTANTIAL PORTION OF SUCH NETWORK OF NARROW FISSURES WHEN THE LIQUID EXPLOSIVE IS PLACED THEREIN OR IN SUCH OTHER ENVIRONMENT AS IT IS TO BE USED. PREFERRED HIGH EXPLOSIVES ARE RDX, HMX AND MIXTURES THEREOF. TNT, PETN OR ANY OTHER HIGH EXPLOXIVE ORGANIC NITRO COMPOUND MAY BE INCLUDED IN AN AMOUNT SUFFICIENT TO RENDER THE LIQUID EXPLOSIVE LESS SENSITIVE TO DETONATION, SUCH THAT IT IS NOT CAP DETONABLE, AT THE SAME TIME ENHANCING ITS EXPLOSIVE POWER AND RELIABILITY. FOR CERTAIN APPLICATIONS, AMMONIUM NITRATE MAY BE ADDED TO ACHIEVE DESIRED EXPLOSIVE EFFECTS, AS WELL AS FINELY DIVIDED REACTIVE METAL TO INCREASE THE BRISANCE OF THE EXPLOSIVE. A GELLING AGENT SUCH AS NITTRECELLULOSE IS INCLUDED TO MAINTAIN THE RESULTING UNIFORM DISPERSION FOR LONG PERIODS OF TIME. A METHOD OF PRESSURE-TRANSFERRING A LIQUID EXPLOSIVE INTO THE WELL BORE AND PRESSURING IT BACK INTO THE PRODUCTIVE FORMATION IS DESCRIBED IN WHICH THE EXPLOSIVE IS INJECTED THROUGH A TUBE INTO THE WELL BORE DIRECTTLY ADJACENT THE FORMATION TO BE FRACTURED, THE TUBE HAVING FIRST BEEN CLEARED OF AIR, BY PLACING THE LIQUID EXPLOSIVE IN ONE OR MORE TANKS CONNECTED TO THE INJECTION TUBE AT THE WELL SURFACE AND WHICH ARE SUBJECTED TO AIR PRESSURE TO FORCE THE EXPLOSIVE INTO THE WELL. FURTHER, A WELL FRACTURING METHOD WHICH IS SELF CLEANING, RENDERING UNNECESSARY THE USUAL CLEANING STEP WHICH FOLLOWS FRACTURING, IS DISCLOSED IN WHICH THE WELL BORE IS RESTTRICTED ABOVE THE LEVEL OF THE EXPLOSION AND IS CLOSED ABOVE THE RESTRICTION WITH SANE OR THE LIKE SUCH THAT SUFFICIENT BACK PRESSURE IS MAINTAINED IN THE WELL FOR SATISFACTORY FRACTURING, WHILE THE RESULTANT GASES ARE SUBSEQUENTLY VENTED THROUGH THE RESTTRICTION, BLOWING THE SAND OR OTHER BALLAST AS WELL AS THE RUBBLE GENERATED BY THE EXPLOSION OUT OF THE WELL BORE.

July 23, 1974 N. ROBERTS LIQUID EXPLOSIVE FOR WELL FRACTURING 2 Sheets-Sheet 1 Original Filed Oct. 4, 1968 C I C o mgoressolr IINVENTOR Leonard N. Robens n E l v 4,0 46 O u //fl A Q 1 0 1 .W

2 Sheets-Sheet 2 Leo nerd N. R0 berfs S R E B O R N L LIQUID EXPLOSIVE FOR WELL FRACTURING Origiml Filed Oct. 4, 1968 Detonator 3,825,452 Patented July 23, 1974 Ser. No. 109,774

Int. Cl. C06b 1/04 US. Cl. 149-38 12 Claims ABSTRACT OF THE DISCLOSURE A cap insensitive liqud explosive is described which is' particularly useful for fracturing a formation containing a network of narrow fissures adjacent a well bore in order to bring in a well or to increase its productivity, which includes a nitroparaffin compound, preferably nitromethane, capable of dissolving substantial amounts of high explosive compounds, and one or more of certain high explosive compounds dissolved therein. The high explosive compounds are of a kind and are present in an amount capable of rendering the liquid explosive sufficiently diameter insensitive to permit propagation of an explosion throughout a substantial portion of such network of narrow fissures when the liquid explosive is placed therein or in such other environment as it is to be used. Preferred high explosives are RDX, HMX and mixtures thereof. TNT, PETN or any other high explosive organic nitro compound may be included in an amount sufficient to render the liquid explosive less sensitive to detonation, such that it is not cap detonable, at the same time enhancing it explosive power and reliability. For certain applications, ammonium nitrate may be added to achieve desired explosive effects, as well as finely divided reactive metal to increase the brisance of the explosive. A gelling agent such as nitrocellulose is included to maintain the resulting uniform dispersion for long periods of time. A method of pressure-transferring a liquid explosive into the well bore and pressuring it back into the productive formation is described in which the explosive is injected through a tube into the well bore directly adjacent the formation to be fractured, the tube having first been cleared of air, by placing the liquid explosive in one or more tanks connected to the injection tube at the well surface and which are subjected to air pressure to force the explosive into the Well. Further, a well fracturing method which is self cleaning, rendering unnecessary the usual cleaning step which follows fracturing, is disclosed in which the well bore is restricted above the level of the explosion and is closed above the restriction with sand or'the like such that sufficient back pressure is maintained in the well for satisfactory fracturing, while the resultant gases are subsequently vented through the restriction, blowing the sand or other ballast as well as the rubble generated by the explosion out of the well bore.

This application is a division of my application Ser. No. 765,113, filed Oct. 4, 1968, now abandoned; a continuation of that application filed Ian. 27, 1971 under Ser. No. 110,315 issued as. United States Pat. No. 3,659,652 on May 2, 1972.

BACKGROUND OF THE INVENTION 1. Field of the invention This invention relates to a liquid explosive which is particularly suitable for fracturing a geological formation adjacent a well bore, for bringing in the well or for increasing the productivity of a well which has substantially ceased to produce oil, water or gas, and to a method of using the explosive for that purpose. The liquid explosive, particularly certain embodiments described herein, is suitable for other applications, such as quarrying, especially where an explosive composition is required which will conform to the formation in which it is placed, which is not adversely affected by oil, water or other geological fluid normally present, which has a high explosive power and which is a class B explosive, this is, it is insensitive to detonation by a No. 8 blasting cap (cap insensitive). The liquid explosive disclosed herein is especially suited to well fracturng, however, for which these properties are highly desirable, and because of its ability to propagate an explosion through a network of narrow fissures in a geological formation. The term narrow fissures or fine fissures as used herein means those fissures which may be created in geological strata adjacent well bores, commonly by hydraulic fracturing, and having widths from approximately inch down to fractions of a millimeter.

To bring in a well, after it has been drilled it is usually necessary to increase the permeability of the producing formation to stimulate flow in the well. This has commonly been done by shooting the well with a nitroglycerin charge, acidizing (in certain types of formations) or hydraulic fracturing. Similarly, when a formerly productive well has ceased to produce, the pay zone may be fractured to reactivate the well. The purpose of fracturing is to increase the permeability of the productive formation, or pay zone, permitting'flow from the producing formation into and up the well bore.

Explosive fracturing was originally carried out by placing a nitroglycerin charge in the well bore and detonating it. The disadvantages of nitroglycerin, used for many years for this purpose, are many. For example, it is extremely shock sensitive and difficult to handle in transport; it is too sensitive, for example, to be pumped or poured into a well and must be carefully placed there. Solid explosives have also been used, but cannot be made to conform to the well bore, let alone the productive formation, and consequently are of limited effectiveness. Liquid and slurry explosives other than nitroglycerin have been tried but in general have not been successful, for reasons including instability, segregation of constituents, detonation problems and vulnerability to leaching and dilution by fluids in well bores.

In explosively fracturing a well, the explosive, solid or liquid, is usually merely placed in the well bore and detonated. If no prior treatment is given the well, this results in some increase in the permeability of the formation. Because the explosive is so localized in the well bore, it is undesirably ineffective in producing an increase in permeability. Consequently, the practice has developed of initially hydraulically fracturing the productive formation in order to create a network of narrow fissures therein, so that these fissures to some extent channel the force of the subsequent explosion further back into the formation, somewhat distributing its effect. In some cases, part of the explosive is forced back under pressure into the fissures created by the hydraulic fracturing prior to detonation, obtaining a greater distribution of its effect. However, pumping explosives into wells under pressure, back into the formation, is always a hazardou procedure and special safety equipment is usually required in order to safeguard the operating personnel. Commonly, pumping and handling equipment at the surface of the well are controlled from a remote site, so that should a detonation accidentally take place, personnel will not be endangered.

In addition, when an explosive is pressured back into the formation, the resultant detonation creates large amounts of rubble and debris, necessitating an extensive cleaning operation by conventional techniques after the detonation. Some of this resultant debris, particularly the finer particles, finds its way back into the fissures and cannot be completely cleaned out, thus materially limiting the increase in permeability which can be expected with this type of fracturing.

2. History of the Prior Art To overcome these drawbacks, experiments have been conducted for several decades with liquid explosives other than nitroglycerin and with slurry explosives, which are dispersions of solid explosives or of one or more explosive constituents suspended in water or some other medium. Liquid (including slurry) explosives have the advantage of being able to conform to and thus more readily fill the well bore, resulting in greater explosive power. It is important that explosives of this kind be capable of being pressured back into the geological formation adjacent the well bore in order to obtain complete, even and adequate fracturing of the formation and to minimize damage to the well bore and to any casing installed in the well.

A serious problem in liquid and slurry explosives developed to date has been their inability to undergo pressurization into a well formation, and still be capable of consistent and reliable detonation without the necessity of using complex and expensive detonating systems. In certain instances indispensible constituents of the explosive are filtered out in passing through the narrow fissures and pores of the formation. In other cases, exposure to fluids in the well bore or formation causes dilution of the explosive, rendering it incapable of detonation, or leaches out certain of its essential constituents.

Other explosive compositions are highly diameter sensitive, meaning that they are incapable of being detonated in spherical volumes of less than a certain diameter. Diameter sensitivity is a measure of the capability of an explosive compound to propagate an explosion in narrow passages such as geological fissures. Diameter sensitivity as used herein has reference to the ability of a composition to propagate an explosion along a tube filled with the composition, containing a restricted orifice of a given diameter, so that the explosion propagates past the orifice and is not extinguished by the reduced diameter of the composition. Thus, an explosive with a diameter sensitivity of 1 inch, placed in a tube of greater diameter will propagate an explosion past a 1 inch diameter orifice but is incapable of propagating an explosion past an orifice of lesser diameter. This indicates that the same explosive will propagate an explosion in a 1 inch diameter geological fissure.

Reference to this problem is made in US. Pat. No. 3,301,724, issued as recently as Jan. 31, 1967, in which it is stated:

A remarkable property of my inventive compositions is that they are able to propagate even in a small diameter drill hole, such as, for example, three inches in diameter. Many commercially used blasting compositions, such as may be produced from ammonium nitrates and diesel oil mixtures perform well in the mass and will propagate in a large diameter drill hole, such as six inche or larger, but fail to propagate at three inches or four inches diameter.

Clearly, however, even propagation in a 3 inch diameter hole is totally inadequate to permit effective use of such explosives for fracturing well formations. Although explosives exist which are not diameter sensitive, compositions using such explosives have encountered one or more of, the other drawbacks mentioned above rendering them unsuitable for well fracturing applications. Certain of such explosives are so highly unstable as to be dangerous, while others are so insensitive to detonation in well formations that resort must be made to complex arrangements of multiple shaped charge for detonation.

As explained above, remote control for pumping and handling liquid explosives is required for purposes of safety, particularly if the explosive is to be pressured back into the formation, and such safety provisions are necessarily time consuming and expensive. While liquid explosives have been developed which are much safer than nitroglycerin, accepted safety practices generally require that they not be handled by pumps and similar conventional equipment, particularly in the presence of personnel. The reason for this is that pumps have a tendency to overheat unpredictably, resulting in a danger of accidental explosion. Consequently, there has been a need for apparatus capable of safely placing liquid explosives down into the well bore without the elaborate safety precautions currently necessary. It is not satisfactory merely to pour the explosive into the bore, for even with those explosives which, when at rest, are virtually completely impervious to fluids commonly found in well bores, dropping the explosive down through hundreds of feet of such fluids causes it to mix with the fluids, separating and churning it into globules so that the explosive is not deposited at the bottom of the bore in a continuou phase. This makes it impossible to consistently achieve detonation and propagation of an explosion throughout the explosive composition. Suggestions have been made to pump the liquid explosive directly into the well bore through a pipe leading down from the surface. However, no explosive disclosed by prior art of which the applicant is aware has been sufficiently impervious to well fluids, even after being pumped through pipes to a point adjacent the formation, to'be capable of consistent and predictable detonation. Moreover, the pumping equipment used with such pipes necessitates the expensive safety precautions described above.

Another time consuming source of undue expense to the well operator is the cleaning operation, mentioned above, required after the well has been explosively fractured. Although certain of the debris settles inescapably back into the newly created fissures, thus limiting the maximum increase in productivity obtainable, a large part of this debris can be removed from the well, after it has settled, by extending conventional cleaning tools through the entire well bore and bringing them back to the surface, along with debris. Such cleaning operations are at best time consuming, and add materially to the expense of a fracturing operation.

SUMMARY OF THE INVENTION This invention is based on the discovery that nitroparafirn compounds, which are themselves explosives but which are very diameter sensitive, may be rendered sufficiently diameter insensitive to be highly effective for well fracturing by dissolving therein certain high explosive compounds, in particular RDX, HMX and mixtures thereof. It has further been discovered that such compositions, particularly those utilizing nitromethane, which is capable of dissolving large amounts of such high explosives, are not cap sensitive when formulated in accordance with the invention and are not subject to leaching, dispersion or other forms of degradation in the well bore or formation.

An embodiment of the invention particularly suitable for well fracturing applications is a solution of nitromethane saturated with one of the high explosives mentioned above and also saturated with TNT or an equivalent high explosive organic nitro compound. Finely diyided metallic powder may be added, along with a gelling agent, to enhance the brisance of the explosive.

An embodiment of the invention suitable for applications other than well fracturing is a saturated solution in a nitroparaffin compound of one of the high explosives mentioned above, along with sufficient TNT to render the composition cap insensitive as Well as from about 0.50% finely divided ammonium nitrate. Finely divided metallic powder may also be added. A gelling agent, such as nitrocellulose, is included to maintain the constituents in stable suspension. It has been found that explosives according to the invention containing ammonium nitrate are unsuitable for use in applications requiring pressurization of the explosive through well formations containing fine fissures, since the fissures tend to filter out the ammonium nitrate, becoming clogged and preventing pressurization therethrough of the liquid explosive. However, such compositions are suitable for other applications, such as quarrying.

As previously explained, fine fissures created through hydraulic or equivalent fracturing in geological formations range generally from /4 inch down to submillimeter levels. The diameter sensitivity required in an explosive compound for any given application depends upon the width of the fissures in which it must propagate an explosion. The width distribution of fissures in a given formation varies depending on the type of initial fracturing used to create the fissures and on the nature of the geological formation, and the diameter sensitivity of the explosive to be used should be chosen accordingly. It has been found that the explosive compound need not be capable of propagating an explosion back through all of the finest fissures in the formation, for highly effective fracturing, but should be capable of propagating an explosion throughout a substantial part of the formation. It will thus be seen that, in general, the smaller the diameter sensitivity of the explosive utilized (i.e. the smaller the diameter through which it will propagate an explosion) the better the explosive will be for a given well fracturing application.

Further, in order to prevent globulation and intermixing of the explosive with well fluids, and to eliminate the need for expensive safety measures, in accordance with the present invention the liquid explosive is injected by special pressurization apparatus through an injection tube leading down from the surface to a point adjacent the formation to be fractured. The tube is connected to at least one, and preferably two or more tanks located at the surface containing the liquid explosive. The tanks are connected to the injection tube through Nalves, and air pressure is applied successively to the tanks in order to force their contents down through the injection tube to the formation. While one of the tanks is being emptied in this manner, the remaining one may be refilled with liquid explosive, permitting continuous loading of the explosive into the well. In this manner, the use of pumping and other equipment having a dangerous tendency to overheat is completely avoided. Further, in accordance with the present invention, a self-cleaning method for explosively fracturing a well is provided in which after the liquid explosive has been loaded into the well bore, a restrictive orifice is placed in the well bore above the level of the explosive, and the bore is loaded above the restrictive orifice with sand or rocks or the like such that, when the explosive is detonated, suflicient back pressure is maintained in the well for satisfactory fracturing, while the resultant gases are subsequently permitted to vent through the restriction blowing the sand or other ballast as well as the rubble generated by the explosion out of the well bore.

DESCRIPTION OF THE DRAWINGS The invention will be described in conjunction with the accompanying drawings, in which FIG. 1 is a schematic sectional view of a well bore, the upper portion of which contain a cement casing and an injection tube extending down to a potentially productive formation and connected to pressurization apparatus at the surface;

FIG. 2 is a schematic sectional view similar to FIG. 1 showing a packing plug set in the well casing and a high pressure pump connected to the injection tube;

FIG. 3 is a schematic sectional view of a well bore showing a restrictive orifice placed in the bore and covered with ballast to achieve self cleaning; and

FIG. 4 is a schematic sectional view of a well bore illustrating the self cleaning effect of the restrictive orifice shown in FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 1. Liquid Explosive The liquid explosive of this invention comprises a nitroparaffin compound containing dissolved high explosives of a kind and in an amount capable of rendering the liquid explosive sufficiently diameter insensitive to permit propagation of an explosion therethrough when placed in narrow fissures (defined above) in a geological formation. Sufficient TNT or equivalent organic nitro explosive compound is preferably added to assure that the explosive is cap insensitive as well as capable of reliable detonation. Finely divided reactive metal may be added to increase the brisance of the explosive. A gelling agent is preferably included to maintain undissolved solids in a stable, even suspension. For certain applications, ammonium nitrate may be added to enhance the amounts of gas generated by the explosive.

The solvent for the liquid explosive described herein is a nitroparaffin compound, particularly nitromethane, nitroethane, nitropropane, or mixtures thereof. Such nitroparafiin compounds are explosive and are not readily soluble in or desensitized by Water, oil or other fluids commonly found in well bores.

According to the invention, one or more organic high explosive compounds are dissolved in the nitroparaffin compound, which are of a kind and are present in an amount capable of rendering the liquid explosive sufiiciently diameter insensitive to permit propagation of an explosion through the liquid explosive when placed in the formation in which it is to be used.

Only certain high explosives are capable of diameter desensitizing nitroparaffins to render them suitable for well fracturing, including RDX (cyclotrimethylenetrinitramine) and HMX (cyclotetramethylenetetranitramine). The precise mechanism whereby particular explosive compounds render nitroparaflins diameter insensitive (and which would explain why others do not) is not known. However laboratory experiments to date suggest that, possibly, the high explosive compounds capable of diameter desensitizing nitroparafiins are those Which dissociate in solution to form ionic nitramine compounds, which compounds may sensitize the nitroparafiin in the desired manner. It may thus be the nitramine-sensitized nitroparafiin in conjunction with the explosive compound itself which produces the desired result. Consequently, it is believed that RDX, HMX, mixtures thereof and any other high explosive which forms ionic nitramine compounds with a nitroparaffin, fall within the class of high explosives suitable for use herein.

It has been found that TNT alone is incapable of diameter desensitizing the nitroparafiin solvent to render it suitable for well fracturing applications. However, TNT is preferably dissolved in the nitroparaffin solvent in addition to the RDX and/ or HMX high explosive compounds in order to reduce the cost of the resultant explosive and provide more energy, as well as to render the explosive cap insensitive. Instead of (or in addition to) TNT, any high explosive organic nitro compound, for example, PETN, may be used for this purpose, and when so used is referred to herein as a high explosive additive to distinguish it from the high explosive compounds described above which form nitramine compounds.

It has been found that in addition to cap desensitizing the liquid explosive, the high explosive additive increases its reliability of detonation. For applications where relability of detonation is required, such as in well fracturing, it is important that the liquid explosive contain a high explosive additive to assure such reliability.

It is important that the nitroparaffin compound used be capable of dissolving amounts of high explosive (RDX and/or HMX) and high explosive additive capable of rendering the resultant liquid explosive reliably detonable and sufficiently diameter insensitive to permit propagation of an explosion therethrough when placed in the narrow fissures found in well formations. In this respect, nitromethane is preferred because it is capable of dissolving greater amounts of high explosive than the C and C nitroparaffins.

In a preferred embodiment of the invention TNT and RDX are dissolved in nitromethane making a saturated solution, so that approximately 10% of the TNT concentration is substituted with RDX. By thus maximizing the amounts of high explosive and high explosive additive present in the liquid explosive, the maximum diameter insensitivity is achieved as well as maximum cap desensitization, reliability of detonation and cost reduction.

From about -20 parts per hundred by weight of the liquid explosive may be finely divided reactive metal such as aluminum, which increases the brisance and power of the explosive. The average particle size of the metal should be below about microns to minimize settling or straining out of the metal and to enhance its reactivity.

For applications not involving well fracturing, or where the explosive need not be pressured back through the formation, ammonium nitrate may be dispersed in the solvent and held in an even suspension by means of a gelling agent. From about 050 parts per hundred by weight ammonium nitrate may be used, depending on the particular application desired. The ammonium nitrate should be finely ground, and may be prepared by milling to an average particle size of about 10 to 20 microns in diameter. This prevents the ammonium nitrate from settling out of suspension and also permits the liquid explosive to be used in geological formations where the fissure size is not unduly small, without the suspended ammonium nitrate being filtered out of the explosive.

In order to retain in suspension all constituents of the explosive which are not in solution, such as ammonium nitrate and aluminum, a gelling agent must be added which is compatible with all constituents. A preferred gelling agent is nitrocellulose, as it is an explosive, in an amount preferably between about 1 and 3 parts by weight. Exemplary of other gelling agents which may be used with nitromethane are a cross-linked guar gum, ethyl cellulose, cellulose acetate and acetate butyrate. While no gelling agent is required for a liquid explosive containing only nitroparaffin and dissolved high explosives, it is beneficial to include a gelling agent even in such compositions to prevent settling out of any constituent which comes temporarily out of solution due to unusual temperature or similar changes. In general, the gelling agent may constitute between about 0.5 and 5 parts by weight of the liquid explosive.

One advantage of the liquid explosive compositions described above is that they are insensitive to detonation by a No. 8 blasting cap, i.e., cap insensitive. In particular, the impact sensitivity of these compositions is greater than 100 centimeters/ 2 kg. weight, as tested according to US. Army Specification titled Military Specification TM-9- 1910, Apr. 14, 1955.

Specific embodiments of the liquid explosive prepared in accordance with the invention are illustrated in the following examples wherein all parts are by weight. The explosives may be prepared by first weighing up the ingredients in the amounts indicated below, and then adding the high explosive and high explosive additive constituents to the nitromethane or other nitroparatfin, preferably in a closed container, raising the temperature to completely dissolve the solids. Then, the nitrocellulose or other gell ing agent may be added and mixed mechanically so that it is completely dissolved. Ammonium nitrate and powdered aluminum or similar metal may then be added until a homogeneous solution is achieved.

EXAMPLE I Constituent: Parts Nitromethane 47.5

TNT 47.5

RDX 5.0

EXAMPLE II Constituent: Parts Nitromethane 47.0

TNT 47.0

HMX 5.0

Nitrocellulose 1.0

EXAMPLE III Constituent: Parts. Nitromethane 42.0 TNT 42.0

RDX 4.2 Aluminum 9.8 Nitrocellulose 2.0

EXAMPLE IV Constituent: Y Parts Nitromethane 25.4 PETN 25.4 RDX 2.5 Nitrocellulose 1.7 Ammonium nitrate 40.0 Aluminum 5.0

EXAMPLE V Constituent: Parts Nitropropane 50.0 TNT 23.0 RDX 2.0 Ammonium nitrate 22.0 Cellulose acetate 3.0

2. Loading the Explosive into a Well intermixed with the bore fluids, forming. globules, so that a continuous phase of explosive is not formed in the bore.. It is believed that this may be one cause of detonation failure or unreliability in previous unsuccessful attempts at fracturing wells in this manner.

Consequently, in accordance with .this invention, the liquid explosive is injected into the formation through an injection tube to a point adjacent the formation to be fractured, preventing it from contacting and intermix-. mg with well fluids on the way down into the bore hole.

This method is partially described in copending US. Application Ser. No. 716,056, filedMar. 26, 1968 by Dr. David A. Fletcher and myself and directed to a Well Fracturing Method and Explosive Slurry for Use Therein. Preferably, the injection tube is constructed so as to permit forcing the air or fluids out of it before sending the explosive through it, preventing them from interfering with the formation of a continuous phase of high explosive in the well bore. 7

FIG. 1 shows a well bore 1 extending into a productive formation 2 containing fissures 3 formed by hydraulic fracturing or the like. Such fissures exist in most wells from the initial fracture used to bring in the well; they may be enlarged by refracturing prior, to the explosive fracture. i

The upper portion of the well bore is provided with a casing 4 lined with concrete 5. An injection tube 6 extends down through the well bore to the productive formation,

which is closed at its lower end and provided with slots 7 about its lower end portion through which explosive may flow into the well bore.

The upper end of the injection tube, which may convenien'tly be formed of pipe having an inner diameter of two inches, is connected through a T-fitting 8 to two pressure tanks 9 and 10 through respective valves 11 and 12. Each pressure'tank is provided with an air pressure gauge 13, Hand an air inlet valve 15, 16 through which it is connected to an 'air compressor 17. A fiowmeter 18 is conne'cted" in the injectiontube to measure the quantity of explosive passing through it.

*Pre'ssure 'is required for loading the explosive into the well'in o'rder'to overcome the bottom pressure of the well. To"loadthe'explosive, one of the tanks 9, 10 is filled (through a fi l Gaps not shown) with explosive and pressuriz'ed by the compressor to force its contents through the injection tube into the well. In order to purge the injection tube of air or fluids which might otherwise mix with the explosive, a wiper plug 19 is first inserted into the injection tube, at any convenient coupling, and is then forced down though the tube by the pressure above it of the liquid explosive. When the wiper plug reaches the bottom of the-injection tube it is trapped against the bottom of the-tube, leaving slots 7 sufficiently exposed to the flowing explosive so that it flows freely into the well bore.

' While one tank is emptying under pressure into the well, the other may be filled so that it may be pressurized and connected to communicate with the injection tube through theappropriate valves when the first tank is empty or, preferably, nearly empty, as indicated by the fiowmeter.

In this manner, a continuous flow of explosive under pressure into the well bore is achieved, without interruption'and without subjecting the explosive to heat and pressure from mechanical pumps which constitute an explosion hazard.

If it is desired to pressurize the explosive back into the formation then after a sufficient amount of explosive has beenplaced'in'the well bore, including that remaining in the injection tube, a second wiper plug 20 is placed in, the top of the injection tube and the latter is disconnected from the pressurization tanks and connected to a high pressure (up to about 2,000 p.s.i.) pump 21. Water is then pumped in under high pressure, filling the injection tube and displacing the liquid explosive ahead of the second wiperplug 20 back into the formation, as shown in FIG. 2.

Before pressurizing the explosive in this manner, however,-a packing plug 22 is placed in the well bore above the production formation, preferably at the lower end of the well casing. Such packing plug 22 may be placed in the well when the injection tube is first lowered into it and may be left open at that time in order to permit air or fluids in the well bore to be displaced upward past the packing plug when explosive is loaded into it, as in FIGJJ. Packing plug 22 is preferably of a type which is sealable; by rotation of the injection tube, with which it is interlocked in a known manner. Thus, before pressurizing the explosive into the formation, the well bore can be'closed off by sealing the packing plug 22 so that pressure built up by the high pressure pump 2.1 will force the explosive back into the formation.

'Whenthesecond wiper plug 20 reaches the bottom of the injection tube, the top is sealed off from communication with the well bore, since the lengths of the first and :second wiper plugs are made so that their sum is greater than the height of the slots 7. By this means, water is prevented from entering the well bore and churning up the-explosive.-After all the liquid explosive has been placedin the well,- pressure is maintained until the internal well pressures have reached equilibrium (about one half hour), thus preventing regurgitation from the formation of the explosive when pressure from above is relieved..,Subsequently, the injection tube and pumping apparatus are removed from the well and the bore is filledswith water or other shock absorbing fluid to a height at least sufiicient to balance any back pressure from the well. A suitable high explosive booster charge is then lowered down the well bore such that it is completely surrounded by liquid explosive. The booster may contain any conventional explosive such as Composition B or nitroglycerin commonly used for this purpose, and the detonator may conveniently include a timer for effecting detonation automatically after a predetermined interval of say, several hours.

Note that the gelling agent present in the explosive must be capable of maintaining an even dispersion of its constituents under conditions of temperature (up to about F.) and pressure (up to about 5,000 psi.) encountered in well fracturing. Finally, a packing plug is seated above the water which has been placed in the bore, and the well is further filled with water; after detonation the packing plug is removed and the well cleaned using conventional techniques.

3. Self Cleaning Explosive Fracturing Method In accordance with the invention, in order to preclude having to clean the well bore as described immediately above, and in order to avoid having rubble from the explosion fall back into the fissures created thereby, unduly li-miting the effectivenes of fracturing, a method is provided for rendering the explosive fracture self cleaning. To this end, instead of sealing the well before detonation or using a conventional packing plug, a packing plug 23 is seated in the well bore as shown in FIG. 3, above the fluid column, which contains about a 3 inch orifice from top to bottom, thereby creating a restriction in the well bore. By adjusting the height of the fluid column below the packing plug, an approximate pressure equilibrium may be maintained within the well. Then, a spherical plug 24 having a diameter less than that of the well bore but substantially larger than that of the orifice in packing plug 23 is placed above the packing plug, and the space above it filled with sand and rock or any other ballast.

The spherical plug 24 acts as a one-way valve, preventing the ballast from falling into the well but permitting pressure from within the well to be relieved upwardly through it. Suflicient ballast should be placed above plug 23 to create pressure within the well adequate to achieve the desired fracturing, yet permitting gases created by the explosion to blow the ballast, the plug and virtually all of the rubble created by the explosion up out of the well bore. Usually several hundred pounds of ballast suffices for this purpose.

FIG. 4 illustrates the self cleaning effect achieved. Within about 15 or 20 seconds of the detonation, during which time the shock wave of the explosion fractures the formation, sufiicient pressure is developed to blow anything within the well bore out above the surface in a geyser, as shown. The packing plug 23 may sometimes be blown out of the bore hole by this explosion.

In accordance with the invention it is desirable to place the maximum amount of liquid explosive possible back into the formation, at the same time leaving sufiicient explosive in the well bore to assure reliable detonation and propagation of the explosion. In this manner, the explosive is distributed substantially spherically through the formation rather than being concentrated in a pool at the bottom of the well bore, thereby optimizing the efficiency of the explosion in fracturing the formation.

The following are examples of a method disclosed herein of fracturing well formations using one of the liquid explosive compositions described in the foregoing examples, as well as examples of self cleaning explosive fracturing. While no information exists with respect to the exact value of diameter sensitivity required to obtain effective fracturing in any given well, excellent results have been obtained generally utilizing an explosive having a diameter sensitivity of approximately & inch.

1 1 EXAMPLE VI A well having a productive formation at a depth of about 2,400 feet and which had substantially ceased to produce was cleaned using a conventional technique to remove paraffins and the like. A liquid explosive having the composition described in Example II above was then placed in pressure tanks connected to an injection tube which extended down to the productive formation, as shown in FIG. 1. A wiper plug was placed in the injection tube ahead of the explosive, and approximately 4,000 pounds of explosive were placed in the well bore. A second wiper plug was then inserted and the injection tube reconnected to a high pressure pump which forced water under pressure through the injection tube, thereby forcing the explosive back into the formation. Before the high pressure pump was turned on, a packing plug previously placed at the bottom of the well casing (which extended down about 1,000 feet from the surface) was sealed to permit pressure to be built up by the pump. After approximately 85% of the explosive had been pressured back into the formation, as determined by monitoring the amounts of water and explosive through the fiowmeter, pressure was maintained for fifteen minutes, after which the injection tube and pumping equipment were removed from the well. A detonator timed for three hours and containing a charge of Composition B was then placed wholly within the liquid explosive remaining in the well bore as determined by sensing the level of the explosive with a thermal probe. The well bore was filled to the bottom of the well casing with water and a packing plug was seated at that height, after which several hundred feet more of water were loaded into the bore over the packing plug. After detonation, the packing plug was removed and resultant debris cleaned out of the well by conventional techniques. Initial tests indicated about a ten fold increase in productivity.

EXAMPLE VII The method of Example VI was carried out using 6,000 pounds of the liquid explosive of Example III. About 80% of the explosive was pressured back into the formation. 'Instead of sealing the well bore with a solid packing plug, a packing plug having three inch diameter hole was seated about 100 feet above the bottom of the well casing, which extended down 1,500 feet from the surface. The depth of the well was 4,000 feet. Water was then placed in the well up to the annular packing plug. This column of water substantially equalized the bottom pressure in the well.

A 9 inch diameter rubber ball was placed over the hole in the packing plug, and approximately 600 pounds of mixed sand and rock were loaded in above it. About 15 seconds after the shock wave from the detonation was first felt, a geyser of debris appeared above the surface of the well, which lasted for close to thirty seconds. Without cleaning, productivity rose from one barrel per day (before fracturing) to 40 barrels per day.

It will be appreciated by those skilled in the art that the exemplary embodiments described above may be modified and still remain within the scope and spirit of the invention, which is limited the following claims. I

I claim: v

1. A liquid explosive for use in fracturing a formation containing narrow fissures adjacent a-well bore to bring in the well or to increase its productivity, whichliquid explosive is non-dispersible in well fluids over a period of time required to fracture the formation, comprising a nitroparaffin compound having dissolved therein a high explosive nitramine compound in an amount capable of rendering said liquid explosive 'sufiiciently solely in accordance with 2. A liquid explosive as defined in claim'l includingan amount of high explosive additive inthe form" of an organic nitro compound dissolved in said nitroparaifin compound sufficient to render the liquid explosive cap insensitive. 1

3. A liquid explosive as defined in claim? wherein said nitroparafiin compound'is nitromethane and said high explosive nitramine" compound is taken from the group consistng of cyclotrimethylenetrinitramine; cyclo-' tetramethylenetetranitramine,and mixtures thereof.-

4. A liquid explosive as defined in claim 2 wherein said high explosive nitramine compoundandsaid high explosive additive are' present in amounts sufficient to saturate the nitroparaffin' compound and including a gelling agent capable of maintaining an even dispersion of any constituents of the liquid explosive which are not in solution at the temperatures and pressures encountered in well fracturing. i

5. A liquid explosive as'defined in claim 3 including from about 0-50% by weight ammonium nitrateev enly dispersed in said nitromethane, and agelling'agentfor maintaining the ammonium nitrate evenly dispersed in the nitromethane. j v

6. A liquid explosiveas defined in claim 3 including from about 0-20% by weight of a finely divided reactive metal evenly dispersed in said nitromethane,'and'a gelling agent for maintaining the reactive metal evenly dispersed in the nitrome'th ane,

7. A liquid explosive as defined in claim 6 wherein said high explosive nitramine compound is cyclotrimethylenetrinitramine, cyclotetramethylenetetranitramine or mixtures thereof and wherein the high explosive nitramine compound and high explosive additive aref'present in amounts sufiicient to saturate the nitromethane.

8. A liquid explosive which is essentially impervious to dilution by fluids normally present in well bores, comprising nitroparaifin and a high explosive nitramine compound dissolved therein consisting of cyclotr imethylenetrinitramine or cyclotetramethylenetetranitramine in an amount capable of rendering said liquid explosive det; onable in diameters smaller than about A 1 I 9. A liquid explosive suitable for fracturing a formation containing narrow fissures adjacent' a well bore"t0 bring in the well or to increase its productivity; which liquid explosive is non-dispersible in well fiuids'fover a period of time required to fracture the formation, comprising a nitroparaffin compound having dissolved therein a high explosive additive in the form of an organic nitro mine compound when dissolved in the nitroparaifin compound, said high explosive compound being present in an amount capable of rendering the "liquid explosive sufiii ciently diameter insensitive to permit propagation of an explosion throughout a substantial portion of such 'network of narrow fissures Whenthe liquid explosive is placed therein. I

10. A liquid explosive asdefin'edin claim 9 including: a high explosive additive inthe form of an organic nitro compound dissolved in said nitroparaffin compound 'in' an amount capable of rendering the liquid explosive cap insensitive. i v

11. A liquid explosive 'as defined in claini 10 wherein said nitroparaffin compound is nitromethaneand said high explosive compound consists of'cyclotrimethylene trinitramine, cyclotetramethylenetetramine or mixtures thereof, ,the high explosive compound and high explosive additive being present 'inamountssutficient to saturate the nitrornethane, and including a gelling agent capable'of maintaining an even dispersion of any constituents of the liquid explosivewhich "are'riot insolutio'n' at the tempera tures'and pressures encountered in well fracturing. I

12; A liquid explosive 'a's defined in claim 11 including from about 020% by' weight ofa finely divided reactive metal powder, fromfabout 050% by weight of finely divided ammonium nitrate and frorn about -05% by Weight of said gelling agent, all of such constituents being evenly distributed throughout the volume of the liquid explosive.

References Cited UNITED STATES PATENTS Wyler 149-91 X Wood 14991 X Maisner 14991 Maisner 14991 STEPHEN J. LECHERT, 111., Primary Examiner US. Cl, X.R. 

